Method and apparatus for the calibration of transducers

ABSTRACT

A calibration system for the dynamic calibration of piezoelectric transducers used in the measurement of blast wave pressure caused by sudden changes of fluid pressure, such as in the firing of various weapons. A calibration chamber is used to create a blast wave of 100 psig upon the transducer being tested. A pressure pulse is generated and supplied to the calibration chamber and the electrical output of the transducer is recorded on a storage oscilloscope to form a first trace. A second trace is generated by a charge amplifier containing a calibration circuit having precision capacitors. The second trace is adjusted to fall upon the first generated trace and the transducer charge sensitivity is read directly from the sensitivity control on the charge amplifier. A series of tests can be used to determine the non-repeatability of the transducer and repetition at different pressures will determine its non-linearity.

[ 51 May 23, 1972 [54] METHOD AND APPARATUS FOR THE CALIBRATION OFTRANSDUCERS [72] Inventor: Charles S. Mangleburg, Colonial Beach,

[73] Assignee: The United States of America as represented bytheSecretary of the Navy [22] Filed: Mar. 23, 1970 [21] Appl.No.: 21,572

Primary Examiner-S. Clement Swisher Attomey-R. S. Sciascia and Thomas 0.Watson, Jr.

[ ABSTRACT A calibration system for the dynamic calibration ofpiezoelectric transducers used in the measurement of blast wave pressurecaused by sudden changes of fluid pressure, such as in the firing ofvarious weapons. A calibration chamber is used to create a blast wave of100 psig upon the transducer being tested. A pressure pulse is generatedand supplied to the calibration chamber and the electrical output of thetrans- [52] U.S.Cl. ..73/4R d r i recorded on a storage oscilloscope toform a first trace. A second trace is generated by a charge amplifiercon- [58] Field of Search ..73/1 A, l 8,4 mining a calibration circuithaving precision capacitors The second trace is adjusted to fall uponthe first generated trace [56] Reta-megs cued and the transducer chargesensitivity is read directly from the UNITED STATES PATENTS sensitivitycontrol on the charge amplifier. A series of tests can be used todetermine the non-repeatability of the transducer 3,273,376 9/1966Aronson et al ..73/4 R and repetition at digerem pressures willdetermine its 3,203,223 8/1965 Petrow linearity. 3,034,332 5/1962Lederer ..73/4 R 2 Claims, 5 Drawing Figures E L ECTRONIC SECTION /0 lBLAST GAUGE CHARGE AMPLIFIER CALIBRATION CHAMBER PRESSURE DIAL PRESSUREPULSE SOURCE a GENERATOR GAUGE VALVING PATENTEDmzamz v 3.664.176

- sum 1 or 2' ELECTRONIC SECTION BLAST GAUGE CHARGE AMPLIFIEROSCILLOSCOPE CALIBRATION CHAMBER PRESSURE DIAL PRESSURE PULSE SOURCE &GENERATOR GAUGE v VALVING Fla. 2

INVENTOR.

CHARLES S. MA/VGLEBURG VA a may ATTORNEY METHOD AND APPARATUS FOR THECALIBRATION OF TRANSDUCERS STATEMENT OF GOVERNMENT INTEREST Theinvention described herein may be manufactured and used by or for theGovernment of the United States of America for governmental purposeswithout the payment of any royalties thereon or therefor.

BACKGROUND OF THE INVENTION The present invention relates generally toimprovements in calibrating transducers, and more particularly itpertains to a new and improved method and apparatus for directlycalibrating piezoelectric crystal transducers without calculation todetermine the transducer charge sensitivity.

In certain types of work it is desirable to know the value of thevoltage that will be developed across the polar faces of a givenpiezoelectric crystal feeding into a given electrical impedance whensubjected to a dynamic fluid pressure of known value. A crystal socalibrated may then be used as a gauge to measure dynamic fluidpressure, such as blast waves caused by the firing of various weaponsand other similar devices in which substantially sudden changes of fluidpressure occur.

Piezoelectric transducers must be calibrated dynamically because they donot have DC response due to the RC decay inherent in them. In the past,it has been the general practice to subject piezoelectric crystaltransducers to a dynamic fluid pressure of known value and record atrace. With this information and by means of calculation the transducercould be calibrated. Such devices have not been completely satisfactorydue to the complexity and inaccuracy of the method, and the need forcomputations in calibration.

The above disadvantages are overcome by the present invention whichprovides a method and apparatus for calibrating a transducer that isquick, simple and accurate and allows the transducer charge sensitivity(picocoulombs per psi) to be determined without the need of calculation.

SUMMARY OF THE INVENTION The general purpose of this invention is toprovide a system for the dynamic calibration of air blast gauges such aspiezoelectric crystal transducers. There are several types of monoaxialcrystals which respond electrically to fluid pressure and are suitablefor use in pressure gauges. One of the most satisfactory of these istourmaline. In the specification herein described, monoaxial crystalssuch as tourmaline, are used in the preferred embodiment, but it is tobe understood that any crystal gauge responsive to dynamic fluidpressure may be calibrated, when suitably mounted, in the manner to bedescribed in the invention.

A calibration chamber is used to create a blast wave of 100 psig uponthe transducer being tested. A pressure pulse is generated and theresulting trace recorded on a storage oscilloscope. A second trace isgenerated by a charge amplifier containing a calibration circuit havingprecision capacitors. The second trace is adjusted to fall upon thefirst generated trace and the transducer charge sensitivity is readydirectly from the sensitivity control on the charge amplifier. A seriesof tests can be used tof the transducer and repetitions at differentpressures will determine its non-linearity.

OBJECTS OF THE INVENTION An object of the present invention is theprovision of a quick, simple and accurate method, and a device therefor,for determining the charge sensitivity of a transducer.

Another object is to provide a method of calibrating a transducer whichrequires no calculation.

Other objects, advantages and novel features of the invention willbecome apparent from the following detailed description of the inventionwhen considered in conjunction with the accompanying drawings.

BRIEF DESCRIPTION OF THE DRAWINGS FIG. 1 is a perspective view of thepreferred embodiment of the invention;

FIG. 2 is a block diagram of the overall calibration system;

FIG. 3 illustrates a cross section of the transducer of the presentinvention;

FIG. 4 shows a trace recorded which represents the response of thetransducer to a pressure pulse; and

FIG. 5 is a cross section of the calibration chamber and pressure pulsegenerator.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT The system shown inFIG. 1 is illustrated in the block diagram of FIG. 2. For conveniencethe system will be described in two sections; electronic and mechanical.The electronic section consists of an air blast transducer or blastgauge 10, a charge amplifier l2 and an oscilloscope 14. The mechanicalsection consists of a pressure pulse generator 16, a calibration chamber18, a dial gauge 20 and a pressure source and valving system 22.

Turning to the electronic section, the transducer or blast gauge 10,shown in FIG. 3, utilizes natural tourmaline piezoelectric crystals 24cut perpendicular to the principal axis and stacked in a four elementpile. The single elements are physically and electrically in parallel.Two elements are located on each side of the transducer and areprotected by a rubber compound 26. A four element pile of one inchdiameter discs produces a nominal sensitivity of 30 picocoulombs per psiwith a nominal capacitance of picofarads. An insert 28 which containsthe elements can be removed from a baffle 30 for calibration.

The charge amplifier l2 chosen for the system was the Kistler InstrumentCorporation Model 503. This amplifiers extremely high input impedancenominally l0 ohms makes it attractive for this application. The designalso simplifies calibration and no calculations are required todetennine the transducer charge sensitivity.

The charge amplifier 12 consists of a charge-to-voltage converter,anadjustable gain voltage amplifier, a calibration or test signalcircuit and a regulated power supply. The chargeto-voltage converter isan amplifier with high input impedance utilizinzcapacitive feedback tomaintain the input near zero voltage. This allows the user tointerchange cables of reasonable length and capacitance withoutappreciably afiecting the transducer charge sensitivity. The voltageamplifier then boosts the output of the charge converter to useablelevels: up to i 10 volts for devices with high input impedance or i 20milliamperes for devices with low input impedance. The output of thecharge amplifier 12 can be adjusted with the gain control of thisvoltage amplifier.

The test signal is produced by a calibration circuit which consists ofan adjustable voltage and a group of precision capacitors. This providesa charge calibration circuit since the voltage applied to one of thecapacitors gives a known charge (Q=C X V). Using a ten turn, linear,calibrated potentiometer (transducer sensitivity control), this testsignal a stable one ducer sensitivity control is set to 10 (full scaleposition), the

full 1 volt is applied to the 10,000 pfd capacitor simulating a 10,000pCb charge. This would correspond to a transducer of 10 pCb per psicharge sensitivity at full scale (1,000 psi). Reducing the transducersensitivity control to three, reduces the voltage to 0.3 volts providinga 3,000 pCb charge. This will correspond to a transducer of 3 pCb perpsi charge sensitivity at full scale (1,000 psi) or a transducer of 30pCb per psi charge sensitivity at 100 psi. Thus an accurately knowncharge signal simulating a piezoelectric transducer at full scale outputis inserted calibrating the charge amplifier 12 from back to front.

Although the complete calibration procedure is discussed below, theoperation of the charge amplifier 12 during calibration will now bebriefly explained. The test signal button is depressed applying thecalibration voltage to the capacitor and the transducer sensitivitycontrol adjusted until the voltage charge obtained is the same as thatof a loaded transducer. The charge sensitivity of the transducer canthen be read directly from the transducer sensitivity control. The gaincontrol does not affect the calibration since both the test orcalibration signals and the transducer signal are amplified equally.

The oscilloscope 14 used in this application was the Tektronix Type RM564 with a type 3A3 vertical amplifier and a type 2867 time base. Thisis a storage type oscilloscope, which in this application saves a greatdeal of time and expense since a permanent photographic record is notrequired. If a photograph is desired, this can easily be accomplished byattaching a camera to the oscilloscope after observation of the tracesince the storage life of the image is quite long.

Turning to the mechanical section, pressure pulse generator 16 is shownin FIG. having a main pressure chamber 36 which rests on a support stand32 having impact weight guides 34. The main pressure chamber 36 (seeFIG. 5) is pressurized with nitrogen or another appropriate gas to thepressure desired by an input port 38 with the poppet valve 40 in theclosed position. An impact weight 42 is released and strikes an impactplate assembly 44. The force is transferred to a valve stem 46 whichcauses the poppet valve 40 to open quickly. The pressure then equalizesin the main chamber 36 and the calibration chamber 18 via an orifice 48.Since the main chamber 36 has a much larger volume than the calibrationchamber 18, the change in pressure in the main chamber is small and theprocess is essentially isothermal. A quartz crystal (not shown) locatedin the impact plate assembly 44 generates a voltage which triggers theoscilloscope 14 prior to the onset of the pressure pulse.

The calibration chamber 28 is located around the air blast transducer10. Special consideration was given to keep the volume small and theorifice 48 connecting the two chambers 18 and 36 large to allow thepressure to stabilize as quickly as possible. The insert 28 in thetransducer is removed form its baffle 30 and is held in the calibrationchamber 18 by a set screw. This completes the ground in the electricalcircuit. A standard Microdot bulkhead connector 50 whose shell is inmechanical and electrical contact with the calibration chamber 18 ismounted on the chamber wall 52. A wire 54 from the inner conductor isattached to the signal output lead of the transducer 10. A Microdot lownoise cable 56 is connected between the connector 50 on the calibrationchamber 18 and the charge amplifier 12.

The calibration chamber 18 was designed so that the insert 28 could beinstalled with its smallest cross section perpendicular to the axis ofthe orifice 48 connecting the main chamber 36 with the calibrationchamber 18. This allows the calibration chamber to fill more quicklywhile distributing the pressure equally over both sides of thetransducer 10. It also more closely simulates field conditions.

One complete side wall 58 of the calibration chamber 18 is removable toallow installation of the transducer 10. The side is held in place byscrews 60 and a pressure seal is effected with a rubber 0-ring.

The tubing used in this system is stainless steel to allow for pressuresup to 1000 psig for other transducers. The pressure read out is a 16inch 100 psig dial gauge whose accuracy is 0.1 percent of full scalewith a higher pressure gauge available for pressures up to 1,000 psig.In addition to the normal input and exhaust valves 22, a pressure volumecontrol (not shown) is included which allows the pressure in the mainchamber 36 to be easily and accurately set.

CALIBRATION PROCEDURE Charge amplifier l2 enables the charge sensitivityof the transducer 10 to be determined easily and quickly. The electroniccomponents are connected as shown in FIG. 2 and the sensitivity andrange controls of the charge amplifier 12 are set for a nominal input ofpCb per psi at 100 psig (3,000 pCb). The gain of the charge amplifier isthen adjusted so that the zero and full scale (test signal) outputscover close to full scale on the oscilloscope grid. The trace 64 ispositioned near the bottom of the oscilloscope grid and a main chamberpressure slightly over 100 psig is set. This is necessary since thefinal pressure in the calibration chamber 18 using this technique mustbe 100 psig. The added volume of the calibration chamber 18 reduces theinitial pressure in the main chamber 36 according to the relation P,V PV where F and V are the initial pressure and volume of the main chamber36 and P and V are the pressure and volume of the combined main chamber36 and calibration chamber 18 after the poppet valve 40 is opened andequilibrium is achieved. The initial pressure is determinedexperimentally before beginning the calibration. The tolerance for thefinal pressure is 100 i 0.1 psig which is easily obtained. A pressurepulse is then generated by releasing the impact weight 42 and theresulting trace is recorded on the storage oscilloscope 14. Arepresentative trace 62 is shown in FIG. 4. The displacement of thetrace is then noted at some time '1' when the oscillations have damped.These low frequency oscillations are air pressure oscillations in thecalibration chamber 18. Using the trace locator feature on theoscilloscope 14 with the test signal button of the charge amplifier 12depressed, the charge sensitivity control on the charge amplifier isadjusted so that the displacement of the trace 64 is equal to trace 62caused by the pressure pulse. With the test signal button depressed, thesweep is initiated. This second trace 64 should fall on the pressurepulse trace 62 at T,. The charge sensitivity of the transducer 10 canthen be read directly from the sensitivity control on the chargeamplifier 12. The procedure can then be repeated to determine thenon-repeatability of the transducer 10. This same procedure can berepeated, for example, at 50 and 20 psi to determine the non-linearityof the transducer.

Obviously many modifications and variations of the present invention arepossible in the light of the above teachings.

What is claimed is: 1. A system for the calibration of a transducercomprising: a main chamber adapted to be pressurized; a calibrationchamber in which the transducer to be calibrated is mounted; valve meansconnecting the main chamber to the calibration chamber; means to quicklyopen and close the valve between the chambers to produce a pressurepulse whereby the transducer receives the pressure pulse and generates afirst signal; an amplifier for generating a second signal of known valuecalibrated to represent particular charge sensitivities; meansconnecting said first signal to said amplifier; oscilloscope meansconnected to said amplifier for recording and storing said first signaland displaying said second signal; and control means on said amplifierfor adjusting the second signal to equal the first signal; said controlmeans calibrated to provide a direct reading of the value of the firstsignal whereby the charge sensitivity of the transducer may be easilyand quickly determined. 2. A method for directly calibrating blast wavetransducers comprising:

mounting a transducer in a calibration chamber; pressurizing a mainchamber to a predetermined pressure; releasing the pressure from themain chamber into the calibration chamber thereby generating a pressurepulse signal from the transducer; applying the signal from thetransducer to a storage oscilloscope to produce a first trace;generating a second signal of known value calibrated to representparticular charge sensitivities; applying said second signal to thestorage oscilloscope to produce a second trace;

adjusting said second trace to correspond with said first trace wherebythe charge sensitivity of the transducer may be directly determined.

1. A system for the calibration of a transducer comprising: a mainchamber adapted to be pressurized; a calibration chamber in which thetransducer to be calibrated is mounted; valve means connecting the mainchamber to the calibration chamber; means to quickly open and close thevalve between the chambers to produce a pressure pulse whereby thetransducer receives the pressure pulse and generates a first signal; anamplifier for generating a second signal of known value calibrated torepresent particular charge sensitivities; means connecting said firstsignal to said amplifier; oscilloscope means connected to said amplifierfor recording and storing said first signal and displaying said secondsignal; and control means on said amplifier for adjusting the secondsignal to equal the first signal; said control means calibrated toprovide a direct reading of the value of the first signal whereby thecharge sensitivity of the transducer may be easily and quicklydetermined.
 2. A method for directly calibrating blast wave transducerscomprising: mounting a transducer in a calibration chamber; pressurizinga main chamber to a predetermined pressure; releasing the pressure fromthe main chamber into the calibration chamber thereby generating apressure pulse signal from the transducer; applying the signal from thetransducer to a storage oscilloscope to produce a first trace;generating a second signal of known value calibrated to representparticular charge sensitivities; applying said second signal to thestorage oscilloscope to produce a second trace; adjusting said secondtrace to correspond with said first trace whereby the charge sensitivityof the transducer may be directly determined.